Aerosols in Global Models – Focus on Europe

Aerosols are chemically complex constituents of the troposphere. They affect human health, visibility, tropospheric chemistry, ecosystems and climate. They exert an opposite to greenhouse gases effect on the global atmospheric temperature by decelerating the warming of the climate. Global 3-dimensional models have to account for the atmospheric processes that affect aerosols in the atmosphere, namely emissions, nucleation, condensation, evaporation, coagulation, cloud processing, atmospheric transport, dry and wet deposition and chemistry/climate feedback mechanisms. To describe this complex atmospheric system, simplifications are requested that result in differences in the model simulations of the budgets and the properties of the tropospheric aerosols and of their climatic impact. Such differences are documented in the frame of the international aerosol model intercomparison exercise AEROCOM (AEROsol model inter COMparison project). A major factor of uncertainty in the model simulations is related to the water associated to the particulate phase that can equal or even exceed the dry aerosol mass. Secondary organic aerosols (SOA), chemically formed in the atmosphere, are another major source of uncertainties in the models. SOA modelling is in its infancy and is actually based on oversimplifications due to the gaps in our understanding of the SOA occurrence and fate in the atmosphere. European aerosol budgets and the contribution of the various aerosol components over the Mediterranean to the total aerosol mass have been computed by the 3-dimensional global chemistry-transport model TM in the frame of the EU funded project PHOENICS (Particles of Human Origin Extinguish Natural solar Irradiance in the Climate System). Natural contributions of dust, sea-salt and SOA to the total aerosol mass are shown to be significant over the Mediterranean. Biogenic SOA are computed to be more important during summer and are expected to increase more in the future atmosphere. Challenges for future research and chemistry/climate model development are highlighted.